A data transmission system typically utilizes modems, a first modem typically implementing a modulation process to convert a digital bit stream into an analog signal and transmitting the modulated signal over a communication channel, and a second modem receiving and demodulating that signal to retrieve the communicated digital bit stream. The modulation process typically implements a carrier system having at least a first carrier signal and a first baud rate. Data transmission, particularly over long distance circuits, generally experiences different channel variations and impairments. To achieve optimum performance over a wide variety of channels, the modem has to be able to adapt its baud rate and carrier frequency to best suit a particular channel connection condition.
The baud rate and carrier frequency adaptation can be achieved using off-line probing. In a typical off-line line probing, data transmission is stopped, and a remote modem transmits a predetermined signal. Both channel frequency response and noise power spectrum are measured to form a signal-to-noise ratio (SNR) spectrum. This SNR spectrum is used to determine an equalizer noise power spectrum and a corresponding decoder SNR at each baud rate and carrier frequency combination. The decoder SNR and other receiver parameters such as signal level, etc., are then used to determine a maximum bit rate each baud rate and carrier frequency combination can support. The baud rate and carrier frequency combination that can support the maximum bit rate is an optimum baud rate and carrier frequency. Generally, if precoding or trellis precoding is used in the data transmission, a corresponding precoding gain may also be determined from the equalizer noise power spectrum, and is reflected in the decoder SNR. Remaining operations in baud rate and carrier frequency selection are the same with or without precoding.
However, off-line line probing has certain drawbacks. Off-line line probing is intrusive, needing dedicated line-probing time and a line probing signal. Off-line line probing sometimes may not predict a modem performance accurately due to the fact that line probing and data transmission use different signals and different compensation algorithms for phase and magnitude jitter compensation, non-linear distortion adjustment, echo cancellations, and so forth. There is a need for a modem having an on-line device and method that alleviates at least some of the problems cited.